The present invention relates to a radiographic imaging system whereby a subject is irradiated with radiation and whereby the radiation that has penetrated the subject is detected by a radiation detector and converted into an electric signal to produce a radiographic image based upon the electric signal obtained by the conversion. The present invention relates in particular to a radiographic imaging system having a function of correcting the offset of data in the radiation detector.
Radiographic imaging systems are used in a variety of fields such as medicine where they are used to produce diagnostic images for medical use and industries where they are used to conduct nondestructive tests. Presently, some radiographic imaging systems use a flat panel detector (FPD), which converts radiation into an electric signal, as a radiation detector for detecting the radiation that has penetrated a subject (e.g., X-ray, α ray, β ray, γ ray, electron beam, and ultraviolet ray).
There are two types of methods that use an FPD as a radiation detector: a direct type and an indirect type. The method of the direct type, for example, collects electron-hole pairs (e-h pairs) generated by a photoconductive film such as one formed of amorphous selenium in response to incident radiation and reads out them as an electric signal. To be brief, the direct type directly converts radiation into an electric signal. The indirect type has a phosphor layer (a scintillator layer) formed of a phosphor that fluoresces in response to incident radiation to convert radiation into visible light through that phosphor layer, reading out the visible light with a photoelectric transducer. Briefly, the indirect type converts radiation into visible light and then converts the visible light into an electric signal.
In a radiographic imaging system using such an FPD, the pixels (radiation detection elements) of the FPD are producing a given electric signal (dark output) even when the FPD is not irradiated. The dark output is not necessarily the same among the pixels. Different pixels generate different dark output. Accordingly, a radiographic image obtained using such an FPD is affected by dark output (offset) from each pixel. Offset correction is an image processing performed to remove such an effect.
In one example of offset correction, an image (dark image) is read out from the FPD when it is not irradiated, and the dark image thus obtained is used as offset correction data, alternatively, necessary processing is performed on this dark image to produce offset correction data, whereupon the offset correction data is subtracted from a radiographic image as taken.
The offset of the FPD changes with time due to the temperature variation and other factors. Thus, it is a known practice to renew the offset correction data at a given timing.
JP 2005-283422 A, for example, describes a radiographic imaging system comprising offset correction data registering means for registering offset correction data acquired according to an offset signal read out as correction data from radiation detecting means when it is not irradiated, offset correction data renewal means for renewing the offset correction data registered in the offset correction data registering means by using an offset signal, irradiation/non-irradiation detecting means for detecting irradiation and non-irradiation, and data processing switching means. In this radiographic imaging system, when non-irradiation is detected by the irradiation/non-irradiation detecting means, the data processing switching means causes the offset correction data renewal means to perform renewal processing. And, when irradiation is detected by the irradiation/non-irradiation detecting means while the offset correction data renewal means is performing renewal processing, the data processing switching means causes the offset correction data renewal means to stop the renewal processing, whereupon the data processing switching means causes correction computation means to perform correction processing. Afterward, when the irradiation/non-irradiation detecting means detects non-irradiation again, the data processing switching means causes the offset correction data renewal means to resume the interrupted renewal processing.
As described above, the offset of the FPD changes with time due to the temperature variation and other factors. To perform an appropriate offset correction, therefore, the offset correction data needs to be renewed as appropriate.
Renewal of offset correction data as described above is performed at a given timing, typically each time, for example, a given time elapses (at given intervals) or when the temperature of the radiographic imaging system (or the environment in which it is installed) undergoes a variation exceeding a given value.
Needless to say, the radiographic imaging system also carries out radiographic imaging while renewing offset correction data.
As is known, after radiographic imaging, and, in particular, after radiographic imaging using a great dose of radiation, an electric charge or a so-called residual image remains inside the FPD based on irradiation even after an image is read out. Although a residual image decreases over time, when radiographic imaging is performed with a residual image, the residual image is superimposed upon a radiographic image subsequently taken, making acquisition of an appropriate radiographic image impossible.
As described above, offset correction data is produced using the dark image in the FPD. When the dark image is read out with a residual image left in the FPD, the residual image is superimposed upon the dark image, making it impossible to produce appropriate offset correction data. When offset correction data affected by a residual image is used, an appropriate offset correction cannot be achieved, causing a quality degradation of the radiographic image.